added decoder_plugin_register()

With the functions decoder_plugin_register() and
decoder_plugin_unregister(), decoder plugins can register a
"secondary" plugin, like the flac input plugin does this for
"oggflac".

src/decoder_api.c

@@ -18,6 +18,7 @@
  */
 
 #include "decoder_internal.h"
+#include "decoder_list.h"
 #include "audio.h"
 
 #include "utils.h"
@@ -25,6 +26,16 @@
 #include "playerData.h"
 #include "gcc.h"
 
+void decoder_plugin_register(struct decoder_plugin *plugin)
+{
+	decoder_plugin_load(plugin);
+}
+
+void decoder_plugin_unregister(struct decoder_plugin *plugin)
+{
+	decoder_plugin_unload(plugin);
+}
+
 void decoder_initialized(struct decoder * decoder,
 			 const AudioFormat * audio_format,
 			 float total_time)